The invention relates to voltage regulators using MOS transistors.
Switching regulators and linear regulators are well known types of voltage regulators for converting an unregulated voltage, such as a battery voltage, to a regulated DC voltage of a desired value. One type of switching regulator is a PWM regulator that turns an MOS switching transistor on and off at a certain frequency. Synchronous switching transistors may also be used. In a conventional buck regulator topology, the power supply voltage is intermittently coupled to an inductor, and the inductor conducts a triangular current waveform to recharge an output filter capacitor. The charged filter capacitor provides a relatively constant voltage to the load. A feedback signal, which is typically a divided output voltage, determines when to shut off the switching transistor during each switching cycle. The switch on-time percentage is called the duty cycle, and this duty cycle is regulated so as to provide a substantially constant voltage at the output despite load current changes. There are many types of switching regulators.
A linear regulator, also referred to as a low dropout (LDO) regulator, controls the conductance of an MOS transistor in series between the unregulated power supply and the output terminal of the regulator. The conductance of the transistor is controlled based upon the feedback voltage to keep the output voltage at the desired level.
Switching regulators are generally considered to be more efficient than linear regulators since the MOS switching transistor is either on or off. When a transistor is fully on, such as in saturation or near the edge of saturation, the transistor is a highly efficient switch, and there is a minimum of wasted power through the switch. However, due to the high frequency (e.g., 2 MHz) turning on and off of the MOS switch, substantial current is used by the control circuit when repeatedly charging and discharging the gate. Accordingly, a PWM regulator is not an efficient choice for very low load currents.
A linear regulator, on the other hand, provides a very smooth output by constantly adjusting a control voltage of an MOS series transistor, which is always conducting. Since the series transistor is not being turned on and off at a high frequency, the control current used to vary the charge on the gate of the MOS series transistor is much less than the control current that would be used by a PWM regulator to repeatedly charge the switching transistor's gate for the same load current.
Voltage converters are known that switch between a PWM regulator and an LDO regulator to obtain the highest efficiency under various circumstances. Such converters are found in U.S. Pat. Nos. 6,229,289 and 6,150,798. However, the LDO series transistors in those prior art circuits are large enough to support the maximum rated load current for the converters. So the LDO control circuit must source and sink a significant amount of charge to and from the gate of the large MOS transistor to rapidly control the conductance (related to Vgs) of the transistor.